The design of devices that operate at very low temperatures including, for example, the proposed Superconducting Super Collider (SSC), has brought about the need for the development and selection of materials that will operate effectively at low temperatures. The performance of known materials and newly developed materials proposed for use in cryogenic environments like the SSC must be tested at correspondingly low temperatures. The present apparatus permits the evaluation of the tensile and compressive properties of such cryogenic materials.
Cryogenic support systems have been developed using a technique known as shrink-fitting. See U.S. Pat. Nos. 4,696,169 and 4,781,034, incorporated herein by reference. It is known, for example, that cryogenic support members can be constructed of a non-metallic rod or tube and a metallic end connection assembled to the tube. The metallic end connection comprises a metallic plug which conforms to the shape of the interior surface of the tube and a metallic sleeve that is positioned over the exterior surface of the tube. The plug and the sleeve are shrink-fitted to the rod or tube to produce a connection that is effective under conditions of compression, tension and bending.
The process of shrink-fitting takes advantage of the differences in the coefficients of thermal expansion of the materials forming the tube, plug and sleeve. In the above example for cryogenic support members, the metallic plug can be cooled so that it shrinks in size. The metallic sleeve can be heated so that it expands in size. The cooled metallic plug is inserted into the interior of one end of the tube maintained at ambient temperature, and the warmed metallic sleeve is slipped over the exterior of the tube at the same end. As the three components (plug, tube, and sleeve) reach thermal equilibrium, the plug expands and exerts an outward force against the interior wall of the tube while the sleeve contracts and exerts an inward force against the exterior wall of the tube. The plug and the sleeve thus exert oppositely directed and substantially counterbalanced forces against each other and against the tube. The tube remains structurally stable, and the plug and sleeve remain firmly in place at the end of the tube, forming a secure end connection.
If the coefficient of thermal expansion of the plug is less than the coefficient of thermal expansion of the tube, and the coefficient of thermal expansion of the tube is less than the coefficient of thermal expansion of the sleeve, then the tube, plug and sleeve may be warmed together to the same, or approximately the same, temperature, and the plug and sleeve then shrink-fitted to the tube. As the plug, tube and sleeve are warmed together, the plug expands less because of its lower coefficient of thermal expansion. The warmed plug is fitted into the interior of the tube while the warmed sleeve, in its expanded state because of its high coefficient of thermal expansion, is slipped over the tube. As all three components are cooled, the tube shrinks at a greater rate than the plug, and the sleeve shrinks at a greater rate than the tube. The plug exerts an outwardly directed force against the inner wall of the tube while the sleeve exerts an inwardly directed force against the outer wall of the tube. The forces exerted by the plug and the sleeve substantially counterbalance each other, and a secure end connection is formed.
The present apparatus utilizes shrink-fitting to attach the annular sleeves and plugs to the coaxial inner and outer tubes. Specifically, with respect to the outer tube, an annular sleeve is shrink-fitted over the upper end of the outer tube to provide a means for suspending the outer tube from the stationary frame. An annular plug is shrink-fitted inside the outer tube at the upper end to counterbalance the force exerted by the sleeve. With respect to the inner tube, an annular sleeve is shrink-fitted over the upper end of the inner tube to provide a site for attaching the inner tube to the dome member which transmits the compressive and tensile forces to the material sample. An annular plug is shrink-fitted inside the upper end of the inner tube to counterbalance the force exerted against the tube by the sleeve.